The organic EL device is a self-emitting device, and has been actively studied for their brighter, superior viewability and ability to display clearer images compared with the liquid crystal device.
In 1987, C. W. Tang et al. at Eastman Kodak developed a laminated structure device using materials assigned with different roles, realizing practical applications of an organic EL device with organic materials. These researchers laminated an electron-transporting phosphor and a hole-transporting organic material, and injected the both charges into the phosphor layer to cause emission in order to obtain a high luminance of 1,000 cd/m2 or more at a voltage of 10 V or less (refer to Patent Documents 1 and 2, for example).
To date, various improvements have been made for practical applications of the organic EL device. In order to realize high efficiency and durability, various roles are further subdivided to provide an electroluminescent device that includes an anode, a hole injection layer, a hole transport layer, a light emitting layer, an electron transport layer, an electron injection layer, and a cathode sequentially formed on a substrate (refer to Non-Patent Document 1, for example).
Further, there have been attempts to use triplet excitons for further improvements of luminous efficiency, and use of phosphorescent materials has been examined (refer to Non-Patent Document 2, for example).
The light emitting layer can also be fabricated by doping a charge-transporting compound, generally called a host material, with a phosphor or a phosphorescent material. As described in the foregoing Non-Patent Documents 1 and 2, selection of organic materials in an organic EL device greatly influences various device characteristics, including efficiency and durability.
In an organic EL device, the charges injected from the both electrodes recombine at the light emitting layer to cause emission. However, because the hole mobility is faster than the electron mobility, some of the holes pass through the light emitting layer. This causes a problem of lowering efficiency. There is therefore a need for an electron transport material with fast electron mobility.
Tris(8-hydroxyquinoline)aluminum (hereinafter referred to as Alq3), a representative light-emitting material, is generally used also as an electron transport material. However, with slow electron mobility and a work function of 5.6 eV, the material cannot be said to have sufficient hole blocking performance.
Insertion of a hole blocking layer is one method of preventing the passage of some of the holes through the light emitting layer and improving the probability of charge recombination at the light emitting layer. Examples of the hole blocking materials proposed so far include triazole derivatives (refer to Patent Document 3, for example), bathocuproin (hereinafter referred to as BCP), and a mixed ligand complex of aluminum [aluminum(III) bis(2-methyl-8-quinolinate)-4-phenylphenolate (hereinafter referred to as BAlq)] (refer to Non-Patent Document 2, for example).
On the other hand, 3-(4-biphenylyl)-4-phenyl-5-(4-t-butylphenyl)-1,2,4-triazole (hereinafter referred to as TAZ) is proposed as an electron transport material having excellent hole blocking performance (refer to Patent Document 3, for example).
TAZ has a large work function of 6.6 eV and a high hole blocking ability, and is thus used as an electron-transporting hole blocking layer laminated on the cathode side of a fluorescent layer or a phosphorescent layer produced by methods such as vacuum vapor deposition and coating. TAZ thus contributes to attaining the high efficiency of an organic EL device (refer to Non-Patent Document 3, for example).
However, TAZ has a big problem of poor electron transportability and needed to be combined with an electron transport material having higher electron transportability for the production of an organic EL device (refer to Non-Patent Document 4, for example).
BCP has a large work function of 6.7 eV and a high hole blocking ability. However, the low glass transition point (Tg) of 83° C. makes the thin film stability poor, and the material cannot be said to be sufficiently functional as a hole blocking layer.
Either of the materials lacks film stability, or has a function insufficient to block the holes. In order to improve the characteristics of an organic EL device, there is a need for an organic compound that exhibits excellent electron-injecting/transporting performance with high hole blocking ability, and has high stability in the thin-film state.
Compounds having an anthracene ring structure and a benzimidazole ring structure are proposed as compounds improved in the above aspects (refer to Patent Document 4, for example).
However, while the devices using these compounds for the electron injection layer or/and the electron transport layer have been improved in luminous efficiency and the like, the improvements are still insufficient. Further improvement for a lower driving voltage, higher luminous efficiency, and particularly higher current efficiency are therefore needed.